A geological formation may be penetrated by a borehole for the purpose of assessing the nature of, or extracting, a commodity of commercial value that is contained in some way in the formation. Examples of such commodities include but are not limited to oils, flammable gases, tar/tar sands, various minerals, coal or other solid fuels, and water.
When considering the assessment and/or extraction of such materials the logging of geological formations is, as is well known, economically an extremely important activity.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the conditions prevailing in a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
The acquisition of such data typically makes use of techniques of well or borehole logging. Logging techniques are employed throughout various mining industries, and also in particular in the oil and gas industries. The invention is of benefit in well and borehole logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
In the logging of oil and gas fields (or indeed geological formations containing other fluids) specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that may contain a hydrocarbon-containing fluid such as oil or natural gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence a wide variety of well logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling.
Broadly, in most cases the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question) and, in the case of some logging tool types, images that represent conditions and substances in downhole locations.
An example of a logging tool type is the so-called multi-pad micro-resistivity borehole imaging tool, such as the tool 10 illustrated in transversely sectioned view in FIG. 1. In this logging tool an annular array of (in the example illustrated) eight pads 11 each in turn supporting typically two lines of surface-mounted resistivity electrodes referred to as “buttons” 12 is supported on a series of caliper arms 13 emanating from a central cylinder 14. During use of the tool 10 the arms 13 press the buttons 12 into contact with the very approximately cylindrical wall of a borehole. The borehole is normally filled with a fluid (such as a water-based mud) that if conductive provides an electrical conduction path from the formation surrounding the borehole to the buttons.
Many variants on the basic imaging tool design shown are known. In some more or fewer of the pads 11 may be present. The numbers and patterns of the buttons 12 may vary and the support arms also may be of differing designs in order to achieve particular performance effects. Sometimes the designers of the tools aim to create e.g. two parallel rows of buttons located on the pad one above the other. The buttons in the lower row are offset slightly to one side relative to their counterparts in the row above. When as described below the signals generated by the buttons are processed the outputs of the two rows of buttons are in effect lain over one another. As a result the circumferential portion of the borehole over which the buttons 12 of a pad 11 extend is logged as though there exists a single, continuous, elongate electrode extending over the length in question.
In general in operation of a tool such as resistivity tool 10 electrical current generated by electronic components contained within the cylinder 14 spreads into the rock and passes through it before returning to the pads 11. The returning current induces electrical signals in the buttons 12.
Changes in the current after passing through the rock may be used to generate measures of the resistivity or conductivity of the rock. The resistivity data may be processed according to known techniques in order to create (typically coloured) image logs that reflect the make-up of the rock and any minerals or fluids in it. These image logs convey much data to geologists and others having the task of visually inspecting and computationally analysing them in order to obtain information about the subterranean formations.
In use of a tool such as that shown in FIG. 1 the tool is initially conveyed to a chosen depth in the borehole before logging operations commence. The deployed location may be many thousands or tens of thousands of feet typically but not necessarily below, and in any event separated by the rock of the formation from, a surface location at which the borehole terminates.
Various means for deploying the tools are well known in the mining and oil and gas industries. One characteristic of most if not all of them is that they can cause a logging tool that has been deployed as aforesaid to be drawn from the deployed location deep in the borehole back towards the surface location. During such movement of the tool it logs the formation, usually continuously. As a result the image logs may extend continuously for great distances.
Although the logs are continuous in the longitudinal sense, notwithstanding the pad offsetting explained above they are azimuthally interrupted by reason of the pads not extending all the way continuously around the circumference of the borehole. The design of the tool prevents this since the arms 13 must be extensible in order to press the pads 11 into contact with the borehole wall. Following extension of the arms there exists a series of gaps between the ends of the pads.
No data can be logged in these gaps, which manifest themselves as elongate spaces in the image logs. An example of an image log 16 including several of these gaps or discontinuities 17 is visible in FIG. 2. The discontinuities extend from one end of the reconstructed image log to the other, a distance in some cases of thousands of feet.
Filling in the missing data is advantageous for obvious reasons of the desirability of completeness of information. Moreover it is likely to be required when it is desired to process the image logs using automatic pattern recognition programs in order to try and identify certain features in the logs.